Fuel cells have been identified as a relatively clean and efficient source of electrical power. Alkaline fuel cells are of particular interest because they operate at relatively low temperatures and have a high theoretical efficiency compared to other fuel cell technologies. Acidic fuel cells and fuel cells employing other aqueous electrolytes are also of interest. Such fuel cells operate at a voltage of usually less than one volt (typically 0.5-0.9 V). To achieve higher voltages, fuel cells are typically arranged in stacks. Fuel cells employing a liquid electrolyte typically comprise an electrolyte chamber that is separated from a fuel gas chamber (containing a fuel gas, typically hydrogen) and a further gas chamber (containing an oxidant gas, usually air). The electrolyte chamber is separated from the gas chambers using electrodes that are gas permeable, and carry a catalyst such as platinum. Within a stack of fuel cells the electrolyte may be circulated through the electrolyte chambers from headers or distribution ducts, so that the electrolyte flows through all the cells are in parallel.
A problem with such an arrangement is that there will be some electrical (i.e. ionic) leakage current between one cell and another through the electrolyte in the headers or distribution ducts. This can be minimised by designing the electrolyte flow paths to raise their ionic resistance, but this measure cannot eliminate the ionic leakage currents altogether. Another problem with such fuel cell stacks is to ensure uniformity of pressure and mass flow rates between the cells and within every cell.